1. Field of the Invention
The present invention relates to a sheet processing apparatus that aligns and binds sheet-like recording media such as paper sheets, recording paper, transfer paper, and OHP transparent sheets (in the present specification, simply referred to as sheets) conveyed, an image forming system that includes the sheet processing apparatus and an image forming apparatus such as a copying machine, a printer, a facsimile, or a digital multi-function peripheral (MFP), and a sheet processing method performed in the sheet processing apparatus.
2. Description of the Related Art
There has been a known device referred to as a finisher provided with a stapler that stacks sheets discharged from an image forming apparatus on a staple tray, aligns the sheets in a conveying direction (i.e., vertical direction) and in a direction orthogonal to the conveying direction (i.e., width direction), and then binds the sheets. When the stapler performs side stitching that binds the sheets at an edge face of the sheet, the stapler can move in the direction orthogonal to the sheet conveying direction along the end portion of the sheets (normally, trailing ends of sheets) in a state where the sheets are stopped on reference fences that define the position of the sheets in the conveying direction and are held in the stopped state (hereinafter, referred to as abutting) so as to change a binding position.
However, in a conventional construction of a side-stitching processor, the trailing end of the sheet is stopped on the reference fences to align, and the positional relationship between the reference fences and a staple unit in the conveying direction in an end binding process is fixed. Therefore, it is not allowed to adjust the binding position in the sheet conveying direction to a position desired by an individual user in the end binding process.
Accordingly, for example, Japanese Patent Application Laid-open No. 2008-156073 discloses an invention of a sheet processing apparatus that performs a given process on sheets conveyed and discharges the sheets and that makes a retracting structure of a binding device and a driving source for trailing-end fences unnecessary to achieve space-saving at low cost. The sheet processing apparatus includes an intermediate tray that temporarily receives and stacks the sheets conveyed, the trailing-end fences that abut on the trailing ends of a plurality of sheets stacked on the intermediate tray and conveys the sheets to a transferring position, an ejecting claw that takes over the conveyance of the sheets subsequent to the transferring position from the trailing-end fences and delivers the sheets out from the intermediate tray, and a conveying drive mechanism that drives the trailing-end fences and the ejecting claw by the driving force of a single motor.
Furthermore, Japanese Patent Application Laid-open No. 2009-263127 discloses an invention of a sheet post-processing apparatus that reliably makes the leading ends of sheets abut on leading-end stoppers, so that the alignment can be performed. The sheet post-processing apparatus includes discharging rollers that convey sheets discharged from an image forming apparatus and discharge the sheets to a slantingly arranged aligning tray to stack the sheets, and the movable leading-end stoppers that align the sheets in a conveying direction by making the trailing ends of the sheets abut on stopping surfaces of reference fences after pressing the leading ends of the sheets discharged to the aligning tray, and the pressing amount of the leading-end stoppers in the sheet conveying direction is made variable.
However, when the trailing-end reference fences (leading-end stoppers) in front and back have positional deviations in the conveying direction due to a mechanical error arisen in assembling and such, because the stacked sheets are stacked at an angle, and thus, even when the pressing amount of the leading-end stoppers is made variable, the inclination of the sheets is not eliminated. Consequently, the binding depths in the conveying (sub-scanning) direction are not equal when the sheets are bound on front side and when bound on back side, whereby the binding depths are not aligned to an intended depth.
Accordingly, an object of the present invention is to align the binding positions in the sub-scanning direction regardless of the binding position.